The present invention relates to a method for demodulation of FSK signals and a demodulator for performing such a method, as are respectively described in the non-characterizing portion of claim 1 and claim 7.
Such a method and device are already known in the art, e.g. from xe2x80x9cU.S. Pat. No. 6038268xe2x80x9d. Therein, a direct conversion FSK signal radio receiver is shown whereby, after having deduced from the received signal, the separated two-axis components, processing is done on these components. This operation thereby consists of a differentiation and a multiplication with the other, non-differentiated component, the results of both multiplications then being subtracted such as to obtain the demodulated signal.
A drawback of this method is however its sensitivity to noise.
Indeed, by operating on the instantaneous value of the incoming signal, and its deduced components, the demodulation method is perturbated by all noise present in the baseband signals. As a consequence the instantaneous frequency represented by the two noisy baseband signals differs strongly from the one represented by the noise-free signals.
An object of the present invention is to provide a method and a device of the above known type but which is less sensitive to noise According to the invention, this object is achieved due to the fact that said method includes the steps as described in the characterising portion of claim 1 and that said device is further adapted as is described in the characterising portion of claim 7.
In this way, noise immunity is obtained in two ways: firstly the signal is digitized, allowing for more accurate processing on the digital signal, and secondly the mathematical operation is performed at samples which are spaced apart in the time by the symbol transmission period, instead of by an infinitesimally small distance, as in the prior art. Thereby all noise present at frequencies higher than the symbol transmission speed is averaged out.
Another characteristic feature of the present invention is described in claims 2 and 8.
Thereby, further non-linearities, are thereby alleviated, while still suppressing the noise. The latter is realised by the summation, which as a matter of fact, corresponds to an integration filter function. This will be clearly shown in the describing portion of this document.
By further sampling the resulting symbol stream at the baseband symbol rate, the thus generated output stream corresponds to a more accurate demodulated signal, at the correct rate.
The present invention relates as well to a receiver which includes a demodulator device as is described in the claim 10.
Further characteristic features of the present method and receiver are mentioned in claims 4 and 11.
Offset present in the demodulated symbols is thereby detected and eliminated. By further tuning the local oscillator frequency with the thus obtained offset value, as is mentioned in claims 6 and 13, the offset removal is thereby further improved.